In recent years, a requirement of a backbone network for a transmission service capacity increases sharply. A 100 Gbit/s system has been put into commercial deployment. 400 Gbit/s and 1 Tbit/s have become a next point of interest of operators. How to improve a system capacity, that is, improve spectral efficiency and implement a flexible variable-bandwidth network architecture, while maintaining a high rate gradually becomes a focus of the industry. Orthogonal frequency division multiplexing (OFDM) has been successfully applied on a broadband wired network and a wireless network. Orthogonal frequency division multiplexing is a special form of a multi-carrier modulation technology, and adds high-speed data streams onto multiple low-speed subcarriers. An OFDM signal is generated and demodulated in a digital domain. Spectrums of OFDM subcarriers are orthogonal and partially overlapped. Therefore, high spectral efficiency can be achieved. Modulation and demodulation of the OFDM signal are implemented based on IFFT and FFT respectively. Orthogonal frequency division multiplexing is a multi-carrier transmission solution that has lowest implementation complexity and that is most widely applied. Because of an advantage of an FFT algorithm, OFDM can easily implement modulation and be applied to a high-speed system. In addition, OFDM may further implement a “soft change” of a modulation format, that is, implement a variable modulation format without changing a hardware architecture, thereby easily implementing flexible variable bandwidth control. The OFDM signal has the foregoing advantages. As a key technology of a high-speed flexible network, OFDM gradually becomes a hot topic of research in the industry.
In a wavelength division multiplexing (Wavelength Division Multiplexing, WDM for short) system based on any modulation format, one optical fiber includes tens of or even hundreds of optical channels. As an optical fiber amplifier is used, multi-wavelength optical signals with relatively high power are coupled into one optical fiber and aggregate on a very small cross section. As a result, the optical fiber begins to present a non-linear feature. This becomes a key factor limiting performance of a transmission system. Therefore, research on how to improve non-linear tolerance of the OFDM signal is of great significance.